Not Applicable
Not Applicable
The present invention relates to an apparatus for depositing CdS and CdTe layers on substrates by means of a PVD process (Physical Vapor Deposition Process) in the form of a CSS (Close Spaced Sublimation) process. In particular, the invention is directed to a process for deposition of sublimated materials onto substrates for the production of CdS/CdTe solar cells.
In industrial semiconductor deposition processes and in those processes for manufacturing large-area electronic components CVD processes (Chemical Vapor deposition Processes) are employed for depositing, particularly silicon or dielectrics, such as SiO2, Si3N4. Corresponding devices for mass production have been known. For the production of electronic and optical components, furthermore, selenium layers have been applied by CVD processes since a long time, as will be described in more detail in the following.
For the production of e.g. electroluminescence displays or certain solar or photovoltaic cells, such as CdS/CdTe solar cells, on the other hand, PVD processes are suited which provide thin layers or films by vapor deposition of material from a heated source. In addition to thermal evaporizers utilized, for instance, in the field of large area electronics (e.g. for the production of displays), particularly the CSS technology has been used during the last decades for CdS/CdTe solar cells. The devices hitherto known have not been suited for a continuous production of larger modules, neither with a view to their dimensions nor to their material sources.
The CSS technology as compared to CVE technology (Combination of Vapors of Elements) offers the advantage that, for instance, CdTe can be deposited with very high deposition rate. The microstructure, on the other hand, and the resulting electric properties of the CdTe layer-generated are suited for solar cell technology only if and when the close space or CSS distance between the sublimation material and the substrate is accurately adjusted and maintained and all the components of the source material, in the present case of the dissociated CdTe, are directly transported to the substrate surface for recombination thereon, independently from the remaining system parameters and the conditions elsewhere in the system. The close space distance is generally smaller than a few percent of the substrate dimensions. In order to ensure, moreover, that the condensation process remains limited to the small distance area between substrate and source material and that the desired temperature-depending dissociation pressure is obtained, pressure and temperature of source material and substrate should sensitively be adapted to each other.
When adapting a CSS apparatus to production scales including large deposition surfaces and high throughput, it should be ensured that the apparatus can be integrated in a continuous production line. The possibility of using a large number of adjacent evaporator sources was turned down because of the immense constructional efforts and the necessity of adjusting similar conditions of deposition at all sources. It was furthermore turned down to employ large-volume receptacles and to vary the temperatures thereof during the course of the deposition in order to adapt for deposition rate changes depending on the filling level of the material source or supply. Such temperature changes are hardly manageable the more so as layer formation on the substrate is considerably affected thereby.
In view of long years of experience in connection with the critical deposition parameters for CdS/CdTe solar cells in CSS processes, the present inventors were indeed surprised that notwithstanding the problematic nature as outlined above it was possible to substantially uncouple the source material from the substrate in a spatial and physical manner and to locally displace the CSS distance between source material and substrate out of a source material receptacle in the area between the substrate and the cover provided at the opening of the receptacle of the inventional apparatus. Accordingly, the source material and substrate are actually separated by a multiple of the CSS distance from each other. Thereby, it became possible to operate with practically any receptacle and material supply.
An object of the present invention is to provide an apparatus for depositing CdS and CdTe layers on substrates by means of a PVD process in form of a CSS process, which is suited for a large-size deposition and a continuous deposition process. The object is solved by the subject matter of claim 1. Advantageous further developments are defined in the subclaims.
Based on the solution revealed in claim 1, it is possible to employ a large-surface sublimation source which allows sufficient material supply for a long uninterrupted production period without having to resort to corrective measures which depend on the actual filling level or having to refill the material supply. The cover provided according to the invention uncouples the deposition process from the material supply. Because of the higher temperature of the cover, the substrate does no longer xe2x80x9cseexe2x80x9d the material supply, as was the case in CSS apparatuses hitherto used, but, instead, the substrate rather xe2x80x9cseesxe2x80x9d the cover as sublimation source which thereby acts as physical deposition source and determines the thermal and kinetic behavior of the evaporated or sublimated materials, respectively. Consequently, the cover provides for spatial and physical separation of the material supply from the substrate. Contrary to disturbances and problems otherwise observed even in case of smallest interferences into the parameters of the deposition process, the deposition behavior, nevertheless, does not change. The inventors using the now completely differently dimensioned CSS deposition apparatus were able to successfully produce CdS/CdTe solar cells of high efficiency and output by applying temperature and pressure parameters which had proved of advantage in a typical conventional CSS device. According to the invention, changes of distribution and rate of deposition, otherwise occurring with decreasing filling level in the material receptacle, can successfully be avoided.
As described further above, substrates have been provided with selenium layers for a number of years using selenium which was evaporated in a material receptacle and impinged from the evaporation receptacle upon the material to be provided with the layer. Since as a rule evaporation and vapor exhaust from the receptacles are non-uniform, it has become, and still is, the practice to close the receptacles by a sieve or a perforated plate with through-holes (See, U.S. Pat. No. 5,532,102; DE 24 36 431 A; and WO 91 04 348 A). These holes plates provided a uniform deposition and vapor transport. Clogging of the holes is avoided in that the hole plate is heated in order to evaporize any condensing material. DE 24 36 431 A utilizes a hole plate the temperature of which is about 5 to 30xc2x0 C. higher than the temperature of the material receptacle. A further advantage of the hole plate is that it blocks material spatters. According to U.S. Pat. No. 5,532,102, the hole plate is brought at the beginning of the deposition to a temperature significantly higher than the material receptacle and subsequently is lowered to that of the receptacle or somewhat higher. The holes in the cover are larger at the marginal zones of the plate in order to obtain a more uniform deposition of the vapor passing through the holes. WO 91 04 348 A employs heating means which are distributed over the complete height of an evaporation crucible in order to ensure highly uniform heat distribution. Uniform heat distribution is further promoted by a perforated cover arranged on the crucible, which is heated to a higher temperature relative to the crucible and should also block material spatters and the like. DE 26 53 909 A describes a thermal evaporization for large-surface substrates, which includes a perpendicularly disposed perforated cover. The surface area of the outlet openings of this vertical evaporization source should be less than 30%, preferably 15%, of the total cover surface. Condensations on the cover are avoided in that the cover material automatically heats up during the course of the deposition operation. In this way, reproducible vapor streams e.g. of evaporated silver, may be obtained.
The present invention, however, was not concerned with the problem to direct and distribute the evaporation cloud of a thermal evaporation source into a uniform and reproducible vapor stream onto a substrate using a perforated plate. The present invention, instead, had to solve the problem to precipitate elements having been dissociated by sublimation on a substrate such that the dissociated elements directly recombine on the substrate with a given micro structure of the resulting layer.
Preferred further developments of the present invention are defined in the subclaims. In this connection it is for instance possible to improve the deposition at the substrate edge zone by means of a particular hole pattern. A considerable improvement is, furthermore, accomplished by using a frame heated to a higher temperature than the temperature of the receptacle. Condensations of the sublimated material at the colder, outer upper edge of the material receptacle are thereby avoided, which otherwise would lead, in addition to material loss, also to transport problems. The material loss at the small gap between frame and substrate is, moreover, substantially reduced because this gap can be considerably smaller adjusted than the CSS distance between substrate and perforated cover. Hence, by means of that frame, considerably longer production periods can be obtained since precipitates at the upper edges of the receptacle which cannot be completely avoided anyway, grow more slowly because of the frame and need be removed less frequently. The frame may be a separate part arranged on the receptacle. It may also be integrated in a receptacle rim extending beyond the cover.